1. Field of the Invention
The present invention relates to a process for producing an extra high tensile steel having a yield strength of 1080 MPa or more that has a high strength despite a low carbon content and is excellent in low temperature toughness and stress corrosion resistance in a stress corrosive environment, such as sea water and salt water.
2. Description of the Prior Art
In recent years, an ever-increasing demand for energy has led to a growing interest in ocean development, such as seabed resource development and seabed crustal and geological survey, for the purpose of ensuring a stable supply of the energy, which has activated construction of containers for deep-sea use and research ships for deep-sea use or ideas for construction of seabed petroleum production bases in relation to deep-sea development.
When various containers are used in a deep-sea environment, since a very high pressure is applied thereto, it is required for materials for these containers to have a high degree of toughness and strength from the viewpoint of structure.
In order to cope with the demand for safe, reliable, high-strength and high-toughness materials, the development of a Ni-containing low alloy steel and an improvement in the quality thereof have been effected in the art. For example, proposals have been made on many production processes, such as a process for producing a high-strength and high-toughness steel comprising Ni--Cr--Mo--V with C+1/8Mo+V &gt;0.26 and Cr.ltoreq.0.8Mo as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 56-9358, a process for producing a Ni--Cr--Mo--V--based extra high tensile steel as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-188655, which enables a high strength and a high toughness to be provided in a wide cooling rate range in a quenching treatment, and a process for a Ni-containing steel product, wherein very low P and very low S treatments are effected for the purpose of ensuring a high toughness. These processes are effective for increasing strength or toughness. However, in none of the steels produced by the above-described processes, stress corrosion in an environment that comes into contact with sea water or salt water contemplated in the present invention is taken into consideration, so that it is difficult to say that these steels are sufficiently safe to use.
Therefore, it is required for steel products to have satisfactory resistance to stress corrosion cracking in sea water.
Examples of extra high tensile steel products having a high reliability underwater include a Ni--Cr--Mo--V-based high-toughness and extra-high-tensile steel proposed in Japanese Examined Patent Publication (Kokoku) No. 64-11105, characterized by comprising a Ni-containing steel having lowered N and O contents and capable of satisfying a requirement of Al (%) .times.N (%) .times.10.sup.4 &lt;1.5, which high-toughness and extra-high-tensile steel has a significant effect. In this steel, however, the stress corrosion cracking resistance at the welding-heat affected zone in sea water is inferior to that in the air as compared with the base material, which requires further study regarding improvement in safety and reliability. On the other hand, Japanese Examined Patent Publication (Kokoku) No. 1-51526 proposes a process for producing an extra high tensile steel having an excellent stress corrosion cracking resistance, which comprises subjecting a Ni--Mo--Nb-based steel having a Ni content of 5 to 8% to direct quenching-and-tempering. The strength of the steel product, however, is lower than that contemplated in the present invention. In the production of a thick high tensile steel by the direct quenching-and-tempering process, close control is necessary from the viewpoint of the homogeneity and anisotropy of the quality in the direction of the plate thickness. Further, there is a possibility that the stability of the quality is deteriorated in the widthwise direction and longitudinal direction within the steel plate.
Thus, the conventional extra high tensile steel products have lower stress corrosion cracking resistance particularly at the welding-heat affected zone in sea water than in the air and are produced by processes that are disadvantageous in the homogeneity of the quality in the thicknesswise direction of the thick steel plate and the stability of the quality within the steel plate. That is, a further improvement in both the steel products and production processes has been desired in the art.
In order to solve these problems, the present inventors have previously filed a patent application (see Japanese Unexamined Patent Publication (Kokai) No. 1-230713) that proposes a process for producing a high-strength and high-toughness steel having an excellent stress corrosion cracking resistance. Although the stress corrosion cracking resistance has reached a high level through a lowering in the carbon content, the development of an extra high tensile steel product having a higher strength and a high toughness has been desired in the art.